The present invention relates to a semiconductor memory device and a method for manufacturing the same, and more particularly to improving the reliability of a semiconductor memory device.
In recent years, a ferroelectric memory device has been developed in the art in which the memory cell capacitor uses, in its capacitance insulating film, a ferroelectric material having hysteresis characteristics such as Pb(Zr,Ti)O3, SrBi2Ta2O9, or the like.
In order to realize a ferroelectric memory device, it is most important to develop a structure, and a method for manufacturing the same, with which memory cell capacitors can be integrated together without deteriorating the characteristics thereof. Particularly, a ferroelectric material used in a capacitance insulating film is a laminar oxide containing oxygen atoms therein, and is easily reduced in a hydrogen atmosphere used in subsequent manufacturing steps after forming the memory cell capacitors, thereby deteriorating the ferroelectric characteristics thereof.
For example, along with the miniaturization of semiconductor devices, a tungsten (W) deposition process by a CVD method has been widely employed for filling a contact hole having a large aspect ratio. The W deposition process is based on the reaction represented by Formula 1 below:2WF6+3SiH4→2W+3SiF4+6H2  (1)
The reaction represented by Formula 1 above is performed in a very strong reducing atmosphere. Moreover, after the Al line formation, an annealing step is performed in a hydrogen-containing atmosphere in order to ensure the MOS transistor characteristics. The semiconductor device manufacturing process includes many other steps that generate, or use, hydrogen.
Hydrogen permeates through most of the materials used in a semiconductor device. Therefore, conventional ferroelectric memory devices have taken measures to prevent deterioration of the characteristics of memory cell capacitors during the manufacturing process, for example, by reducing the hydrogen generation or suppressing the reducing atmosphere in subsequent manufacturing steps after forming the memory cell capacitors, or by covering the memory cell capacitors with an insulative hydrogen barrier film. A conventional method for suppressing/preventing deterioration of the characteristics of memory cell capacitors during the manufacturing process by using a hydrogen barrier film will now be described as an example.
FIG. 16 is a cross-sectional view illustrating a first conventional memory cell 1000 designed so as to suppress/prevent deterioration of the characteristics of the memory cell capacitors during the manufacturing process.
The memory cell 1000 includes a MOS transistor Tr used as a memory cell transistor, and a memory cell capacitor C. The MOS transistor Tr includes a gate electrode 1 formed on a semiconductor substrate S, and high concentration impurity diffusion regions 2. The MOS transistor Tr of a memory cell is electrically isolated from the MOS transistor Tr of another adjacent memory cell by a shallow trench isolation region (hereinafter referred to simply as “STI region”) 3. A word line (not shown) is connected to the gate electrode 1, and a bit line 4 is connected to one of the high concentration impurity diffusion regions 2. A first insulative film 5 and a first hydrogen barrier film 8 are formed on the semiconductor substrate S with the MOS transistor Tr formed thereon.
The memory cell capacitor C includes a lower electrode 7 formed on the first hydrogen barrier film 8, a capacitance insulating film 9 made of a ferroelectric material and formed on the lower electrode 7, and an upper electrode 10 formed on the capacitance insulating film 9. The lower electrode 7 is connected to the other one of the high concentration impurity diffusion regions 2 via a contact plug 6 running through the first insulative film 5 and the first hydrogen barrier film 8.
A second hydrogen barrier film 11 is formed on the first hydrogen barrier film 8 and the memory cell capacitor C so as to cover the memory cell capacitor C, and a second insulative film 12 is formed on the second hydrogen barrier film 11. The upper electrode 10 is connected to an Al line 14 via a contact plug 13 running through the second hydrogen barrier film 11 and the second insulative film 12.
FIG. 17 is a cross-sectional view illustrating a second conventional memory cell 1100 designed so as to prevent deterioration of the characteristics of the memory cell capacitors during the manufacturing process.
The memory cell 1100 illustrated in FIG. 17 has substantially the same structure as that of the first conventional memory cell 1000 illustrated in FIG. 16. However, the memory cell 1100 is different from the first conventional memory cell 1000 in that the second hydrogen barrier film 11 is formed over the second insulative film 12.
A CVD method or a sputtering method is typically used for depositing a hydrogen barrier film. However, a gas used in a CVD method often contains hydrogen and thus generates hydrogen or water during the deposition step, thereby deteriorating the capacitance insulating film, which is made of a ferroelectric material. In view of this, in the manufacturing process of such a conventional memory cell as described above, the second hydrogen barrier film 11, which is formed in a step after the formation of the memory cell capacitor C, is formed by a sputtering method, which does not generate hydrogen during the deposition step, using a material such as Al2O3 or TiN, for example.
However, in the first conventional memory cell 1000 illustrated in FIG. 16, the step coverage of the second hydrogen barrier film 11 is poor at an edge portion E of the memory cell capacitor C, as illustrated in FIG. 18. This adversely influences the crystallinity/packing of the second hydrogen barrier film 11 at the edge portion E, thereby resulting in grain boundaries. Hydrogen having passed through the second insulative film 12 of the memory cell 1000 may intrude into the memory cell capacitor C through such grain boundaries. Such hydrogen deteriorates the capacitance insulating film 9, which is made of a ferroelectric material.
In the second conventional memory cell 1100 illustrated in FIG. 17, when forming the contact plug 13 for connecting the Al line 14 and the upper electrode 10 to each other, hydrogen may intrude into the second insulative film 12 through the side wall of the connection hole in which the contact plug 13 is being formed. The hydrogen diffuses through the second insulative film 12 to reach and deteriorate the capacitance insulating film 9, which is made of a ferroelectric material.
As described above, it is very difficult in the conventional memory cells to suppress/prevent deterioration of the capacitance insulating film, which is made of a ferroelectric material.